This invention relates to the reduction of metals in various acid media and, more particularly, to the reduction of ferric iron and hexavalent uranium contained in phosphoric acid produced by the acidulation of phosphate rock.
Most of the world's production of phosphate comes from marine phosphorites, and large deposits exist in Florida and the Western United States. These deposits generally contain from 50 to 200 ppm uranium (0.005 to 0.02%, or 0.1 to 0.4 pounds per ton). Although these concentrations are only 5% to 10% as high as those of commercially mined uranium ores, the vast extent of these deposits has made them of considerable interest as a uranium source for many years. It has been reported, for example, that mineable reserves of phosphate rock in the United States alone contain about 600,000 tons, or more than 1 billion pounds, of uranium.
A large and increasing portion of commercial phosphate production is converted first to a relatively dilute phosphoric acid by the so-called "wet-process" (as distinguished from the furnace process which produces elemental phosphorus by direct reduction of the ore). The producer first manufactures sulfuric acid, then uses it to digest the rock. The chemical reaction forms phosphoric acid and calcium sulfate. The latter is filtered out, providing enormous quantities of gypsum, a waste produce, and leaving an acid stream typically containing about 30% P.sub.2 O.sub.5. Most of the uranium in the original rock shows up in the 30% acid, and various extraction processes have been developed to extract it therefrom. The 30% acid is generally evaporated to about 54% "merchant acid", which is either sold or used to manufacture a variety of products, chiefly fertilizers. The higher the acid concentration, the harder it is to extract the uranium, so the 30% stage is where the uranium extraction must take place. If uranium is not extracted, it ends up as a minor impurity in the various end products.
A number of prior processes have been developed to recover the minor amounts of uranium contained in wet-process phosphoric acid. In many of these processes, any hexavalent uranium is first reduced to the tetravalent state by the addition of iron and then extracted by contacting the acid with an organic extractant, such as a mixture of mono- and di-(alkylphenyl) esters of orthophosphoric acid, which has a high extraction coefficient for uranium in the tetravalent state. As is known, the coefficient of extraction (E.sub.a.sup.o) is a measure of the extraction powder of a reagent and is defined as the ratio of the concentration of uranium in the organic phase to the concentration of uranium in the aqueous phase at equilibrium.
The reduction of hexavalent uranium in wet-process phosphoric acid to tetravalent uranium necessarily requires the reduction of some of the other metallic impurities to lower oxidation states. Iron, for example, must be at least partially reduced from the ferric state to the ferrous state because the oxidation state of the iron tends to control the oxidation state of the uranium. Therefore, to reduce the uranium it is necessary to reduce some or all of the iron in solution, and there is about 60 times as much iron as uranium. Only a small amount of the iron is typically found in the reduced (ferrous) state.
In the past, the ferric iron has been reduced such that the ferrous iron concentration is at least about 10% of the total iron content and preferably 20% or above in an attempt to achieve a good extraction coefficient (E.sub.a.sup.o). We have found that, even when all the uranium is in the tetravalent state, the presence of iron in the ferric state is detrimental to the extraction coefficient of a mixture of mono- and di-(alkylphenyl) esters of orthophosphoric acid. This detrimental effect has been found even when a phenol modifier such as nonylphenol or octylphenol is present which eliminates the precipitation of ferric salt of the mixed ester as disclosed in commonly assigned, copending application Ser. No. 772,818, filed Feb. 28, 1977, for "PROCESS FOR EXTRACTING URANIUM FROM WET-PROCESS PHOSPHORIC ACID" by William M. Leaders et al.
The use of iron metal to reduce the ferric iron in the acid to low levels, for example concentrations less than 1.5 g/l, is normally uneconomical since at these low levels of ferric iron concentration the efficiency of the reaction 2Fe.sup.+3 +Fe.sup.o .fwdarw.3Fe.sup.+2 is low, and much of the iron is consumed in producing hydrogen gas in accordance with the reaction Fe.sup.o +2H.sup.+ .fwdarw.Fe.sup.+2 +H.sub.2 .uparw.. Moreover, the additional iron which is necessarily dissolved in the acid makes the acid more difficult to process by the acid producer into "merchant grade" acid. Other metals, such as zinc, copper, and tin, although effective, must also be excluded as potential reductants on the basis of their cost.
There is also a need in the art for an effective reductant for other metals contained in wet-process phosphoric acid and other acid media. These metals include vanadium, molybdenum and rare earths and the acid media include sulfuric and hydrochloric acid as well as phosphoric acid.
Accordingly, it is an object of the present invention to provide an improved process for reducing metals in acid media.
A further object of the present invention is to provide an improved process for reducing ferric iron and hexavalent uranium in wet-process phosphoric acid.
A still further object of the present invention is to provide an improved process for reducing ferric iron and hexavalent uranium in wet-process phosphoric acid to provide a feed acid which is suitable for extraction with an extractant which is selective for tetravalent uranium.
Still a further object of the present invention is to provide a process for reducing ferric iron and hexavalent uranium in wet-process phosphoric acid to provide a feed acid which is suitable for extraction with mono- and di(alkylphenyl) esters of orthophosphoric acid.
Yet a further object of the present invention is to provide a process for reducing ferric iron and hexavalent uranium in wet-process phosphoric acid which provides a feed acid which can be more efficiently extracted.
A still further object of the present invention is to provide a process for reducing metals in acid media which is economical and minimizes the consumption of costly reagents.